Barchan sand dunes - on Mars and on Earth

The top photo is from NASA's APOD site:

…persistent winds may make large sand dunes appear to flow and even drip like a liquid. Visible on the above image right are two flat top mesas in southern Mars, where the season is changing from Spring to Summer. A light dome topped hill is also visible on the far left of the image. As winds blow from right to left, flowing sand on and around the hills leaves picturesque streaks. The dark arc-shaped droplets of fine sand are called barchans, and are the interplanetary cousins of similar Earth-based sand forms. Barchans can move intact downwind and can even appear to pass through each other.

The lower photo is from National Geographic:

Strange creatures slithering up from the Empty Quarter desert floor in the Arabian Peninsula are really a network of barchan dunes—sculpted by winds that over time strike the sand from a consistent direction.